Doctors today call for UN guidelines to be changed following research showing that exclusive breastfeeding protects the babies of HIV positive women from becoming infected with the virus that causes Aids.

There has been a heated debate over the best advice to give new mothers with HIV. Guidelines from Unicef, the World Health Organisation and UNAIDS say the best option is to bottle-feed the babies with formula milk, where it is safe and practical to do so, given potential problems such as tainted water supplies. Where exclusive bottle-feeding is not possible, mothers should exclusively breastfeed, they say.

But the most widespread practice in sub-Saharan Africa, where the Aids pandemic is at its worst, is mixed feeding - women supplement breastfeeding with formula milk and solids such as porridge.

Research in the medical journal the Lancet today shows that is the worst of all worlds. Babies of mothers with HIV who receive a mixture of milk and solid foods are 11 times more likely to become infected than those who are exclusively breastfed. Those who are given formula milk as well as breast milk are nearly twice as likely to become HIV positive.

The issue is not just HIV. Half the babies in the study were born to uninfected mothers. Yet Hoosen Coovadia, Nigel Rollins from the University of Kwa-Zulu Natal in South Africa, and colleagues found that roughly twice as many babies who received mixed feeds died than babies who were exclusively breastfed.

An earlier series in the Lancet on child mortality found that the immunity conferred by the mother on her child through breastmilk, as well as the avoidance of tainted water or other foodstuffs, gave the baby considerable protection from disease.

Even in countries with high HIV prevalence, it calculated, exclusive breastfeeding could prevent 13% of deaths in children under five years-old.

The KwaZulu Natal study involved around 2,700 babies born between 2001 and 2005. Major efforts were made to encourage and support women in breastfeeding by sending counsellors to their homes twice a week. The success of the strategy surprised the researchers.

The authors say exclusive breastfeeding "ordinarily protects the integrity of the intestinal mucosa, which thereby presents a more effective barrier to HIV".